Method of forming a configuration of interconnections on a semiconductor device having a high integration density

ABSTRACT

A method of the kind consisting in that a contact is obtained with an active zone (11) carried by a semiconductor substrate (10) by means of conductive contact studs (18a) located in the contact openings (16c) of an isolating layer (12) and in that then a metallic configuration of interconnections (22) is formed establishing the conductive connection with the conductive contact studs (18a). A separation layer (13) is provided between the isolating layer (12) and the conductive layer (18), which can be eliminated selectively with respect to the isolating layer (12). Thus, the isolating layer (12) retains its original flatness and the conductive contact studs (18a) have an upper level (20) exceeding slightly the level (21) of the isolating layer (12), thus favoring the contact between these contact studs (18a) and the metallic configuration of interconnections (22). Application in microcircuits having a high integration density.

BACKGROUND OF THE INVENTION

The invention relates to a method of forming a configuration ofinterconnections on a semiconductor device, more particularly anintegrated circuit, this method comprising the following steps:

(a) forming an insulating layer on the substrate, in which the elementsof the device are provided,

(b) etching narrow contact openings into this insulating layer,

(c) depositing at least one layer of conductive material by means of amethod ensuring a good coverage, inclusive of the inner surface of thecontact openings, the overall thickness of said layer being sufficientto fill the volume of the contact openings,

(d) removing by etching the major part of the conductive material toexpose the surface of the insulating layer, but to maintain the materialin the contact openings,

(e) depositing a metallic interconnection layer and etching it into theform of the desired configuration.

The semiconductor technology shows a constant development towards anincreasingly higher integration of the number of elementary parts in thesame monolithic circuit.

For this purpose and in order to increase the speed of operation of thecircuits, there is a tendency to reduce as far as possible thedimensions of the elementary parts. The conventional techniques offorming contacts on the semiconductor devices used until recentlycontact zones and interconnection lines whose lateral dimensions mostfrequently were considerably larger than the thickness of the metalliclayer constituting the said lines. As far as the manufacture of circuitshaving a high integration density is concerned, it is on the contrarynecessary to provide contact openings whose diameter is of the sameorder as the thickness of the insulating layer, in which these contactopenings are formed. In this connection, reference is frequently made tothe ratio between the depth and the diameter of the contact openingsdesignated as "aspect ratio", which, when this ratio is close to 1 oreven larger, indicates that the known techniques of forming contactsbased especially on the simple deposition of an aluminum layer can nolonger be used successfully.

Thus, the method mentioned in the opening paragraph has been proposed toprovide a solution suitable for the formation of a configuration ofinterconnections on a circuit having a high integration density, inwhich the contact openings have an aspect ratio close to or larger than1.

A method of this kind is known from the document EP-A-0 165 185. Amongthe conductive materials intended for filling the contact openings,titanium and tungsten are mentioned. It is otherwise known that aparticularly interesting solution from the viewpoint of performances:especially a low electrical resistance and a good mechanical behaviour,consists in that first a thin layer of titanium-tungsten alloy is usedas adhesion and covering layer for the whole surface and then a thicklayer of tungsten is formed for effectively filling the contactopenings.

The method generally used for depositing the tungsten layer for fillingthe contact openings is that designated as low-pressure chemical vapourdeposition (LPCVD) method.

In fact this method ensures a good coverage of the whole surface fromthe layer of Ti-W alloy serving to activate the nucleation of depositionof tungsten. The filling of the contact openings is obtained when thethickness of the deposited layer is at least equal to half the diameterof these openings.

According to this method, after etching the layer of tungsten in such amanner that only the parts of this layer contained within the contactopenings are maintained, it is observed that the upper surface of theinsulating layer thus exposed frequently has a more or less pronouncedroughness which can be irregularly disturbed over the substrate.

This degradation of the flatness of the surface of the insulating layerseems to be associated with the formation of micro-crystals in the layerof conductive material serving for filling the contact openings. Infact, this layer is necessarily fairly thick and is therefore obtainedin conditions which promote a comparative high rate of deposition insuch a mannner that the method remains economical.

During the step of etching the conductive material, it is commonpractice that the etch employed is not selective with respect to thematerial of the insulating layer and that there is therefore a tendencyto attack a superficial fraction of said insulating layer in order toguarantee the complete elimination of the conductive material everywhereexcept in the contact openings.

The surface roughness is then associated with the insulating layer.

The continuation of the manufacturing process of the semiconductordevice is seriously disturbed by the appearance of this fault. In fact,the surface roughness of the insulating layer influences thecrystallization of the metallic interconnection layer generally made ofaluminum, which subsequently covers it and result in a reduction of theresistance to electromigration of this metallic layer.

On the other hand, the roughness is also reproduced at the surface ofthe metallic layer and during the operation of photomaking this layerleads to a substantial degradation of the optical definition of themasque of lacquer to be formed for locally etching the metallic layer.

SUMMARY OF THE INVENTION

Thus, the invention has for its object to provide an improvement of theindicated method such that the aforementioned difficulties are avoided.

According to the invention, a method of forming a configuration ofinterconnections on a semiconductor device, more particularly anintegrated circuit, comprises the following steps:

(a) forming an insulating layer on substrate, on which the elements ofthe device are provided,

(b) etching narrow contact openings into this insulating layer,

(c) depositing at least one layer of conductive material by means of amethod ensuring a good coverage inclusive of the inner surface of thecontact openings, the overall thickness of this layer being sufficientto fill the volume of the contact openings,

(d) removing by etching the major part of the conductive material toexpose the surface of the insulating layer, but to maintain material inthe contact openings,

(e) depositing a metallic interconnection layer and etching it into theform of the desired configuration,

and is characterized in that, before the contact openings are etched(before the aforementioned step b), the insulating layer is covered by aso-called separation layer of such a kind that it can be etchedselectively with respect to the insulating layer and in that openingscorresponding to the contact openings are etched into the separationlayer, whereupon the steps of method indicated at b, c and d areeffected, and in that, before the metallic interconnection layer isdeposited (step e), the separation layer is selective eliminated.

Since the separation layer is eliminated selectively with respect to theinsulating layer, not a single surface irregularity that may be carriedby the separation layer can be transmitted by etching to the surface ofthe insulating layer, which retains its original flat state.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1, 2, 3a, 4a and 5a show diagrammatically and in sectional a partof the semiconductor during various successive stages of the methodaccording to a first embodiment of the invention;

FIGS. 3b, 4b and 5b show views similar to those of FIGS. 3a, 4a and 5a,but of a second embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to an advantageous embodiment of the invention, the separationlayer is chosen so that it can be eliminated selectively with respect tothe conductive filling material. Thus, by the value of the thicknessgiven to the separation layer there is a parameter which permits ofadjusting after removal of the major part of the conductive material theupper level of the remaining parts of this material in the contactopenings with respect to the level of the surface of the insulatinglayer. It has in fact been found that the quality and the regularity ofthe contacts obtained by the aforementioned method depended for asubstantial part upon the fact that the level of the parts of theconductive material is not lower than the level of the adjacentinsulating layer and that these parts of conductive material on thecontrary form a protuberance having a controlled value, whichfacilitates a low-resistance contact with the metallic layersubsequently covering it.

According to a first detailed embodiment of the invention, the method ischaracterized in that the insulating layer is formed by a silica glass,the filling material is formed by tungsten or an alloy rich in tungstenand the separation layer is formed by silicon nitride.

In this case, with respect to the choice of the thickness of theseparation layer, the superficial fraction of this layer which will beeliminated during the continued step of etching the tungsten layer whenthis attack is slightly prolonged in order to take into account smalloperating tolerances, which are in practice inevitable, should be takeninto account.

The etching selectivity of silicon nitride with respect to silica glasspermits eliminating the separation layer, which leaves the surface ofthe insulating layer in its original state, i.e. devoid of roughness.

According to a second embodiment of the invention, the method ischaracterized in that the separation layer is chosen to be of such akind that the conductive filling material can be etched selectively withrespect to the separation layer, which is used during the step ofetching the conductive material (step d of the method) as anetch-stopping layer.

According to this embodiment, the separation layer is practically notconsumed at the end of the elimination of the conductive material andthe elimination of the roughness of the surface is obtained from thisstage of the method.

According to an advantageous variation of this second embodiment, duringthe step of etching the contact openings, the separation layer is etchedselectively, this layer serving as an additional mask for etching theinsulating layer. Since precisely contact openings should be formedhaving a high aspect ratio for which practically vertical walls of theopenings should be obtained, advantageous anisotropic attackingconditions can be chosen, in which the separation layer plays the partof a mask having a higher resistance than the mask of photoresist usedin initially for defining the openings in the separation layer.

According to a preferred variation of the second embodiment of theinvention, the method is characterized in that, the filling conductivematerial is formed by tungsten or an alloy rich in tungsten and theseparation layer is formed from one of the metals aluminum, an alloy ofaluminum or cobalt.

In order that the invention may be readily carried out, it will now bedescribed more fully, by way of example, with reference to the figuresaccompanying drawing.

The method according to the invention relates to the manufacture of aconfiguration of interconnections on a semiconductor device, of which apart shown diagrammatically in FIG. 1.

The different active elements of an integrated circuit have been formedin a substrate 10 of, for example, silicon, the different stages of themethod to be described serving to form an electrical contact with anactive region 11 of the device, which may be, for example, a source ordrain region of a field effect transistor of the MOS type. An insulatinglayer 12 most frequently made of silica glass that may be doped withphosphorous or boron is formed on the substrate 10.

In order to ensure a suitable insulation of the configuration ofinterconnections which will subsequently be carried by the insulatinglayer 12 and in order to reduce to a minimum the parasitic capacitanceof this configuration with respect to the substrate, the insulatinglayer 12 has a comparatively large thickness, of the order of 0.8 to 1μm.

According to the method of the invention, the insulating layer 12 iscovered with a so-called separation layer 13 of such a kind that it canbe etched selectively with respect to the insulating layer 12. Inaccordance with the material chosen to form the separation layer 13, thethickness of this layer can be varied, but nevertheless it generallylies between 50 nm and 300 nm. The assembly of the device has then beencovered with a photoresist mask 15, in which openings 16a are formed atthe areas at which contacts have to be provided.

As is shown in FIG. 2, by means of the mask 15 an opening 16b is etchedinto the separation layer 13 and then an opening 16c is etched into theinsulating layer 12, the opening 16c having a depth such that thesurface of the active region 11 is esposed. In order to limit as far aspossible the volume of the elements of the integrated device, thediameter of the contact openings, such as 16c, is, for example, 0.8 μmso that these openings have an aspect ratio equal to or slightly largerthan 1. In order to obtain contact openings 16c, whose walls arepractically vertical, an anisotropic etching method is used which isknown as "reactive ion etching". For etching the opening 16b into theseparation layer 13, use is made of an etching technique directlyassociated with the kind of the material chosen to form this layer,further details about this technique being given below.

However, it should be noted that the separation layer 13 has a thicknesssuch that the opening 16b formed therein has an aspect ratioconsiderably smaller than 1 and that consequently the angle of the wallsof this opening 16b is not so important as for the opening 16c in theinsulating layer 12.

Reference is now made to FIG. 3A, which corresponds to the stage of themethod at which a layer of conductive material 18 is deposited having asufficient thickness so that the contact openings 16b, 16c are entirelyfilled with this conductive material. For this purpose, a method is usedknown to ensure a good coverage of the surface, inclusive of the innersurfaces of the contact openings. Among these known methods is theso-called low-pressure chemical vapour deposition (LPCVD) method oratmospheric pressure chemical vapour deposition method (CVD). Asconductive material 18 tungsten, an alloy of tungsten an titanium, analloy of tungsten and silicon or highly doped polycrystalline silicon tobe sufficiently conducting can be chosen.

The following operation consisting in removing by etching the major partof the conductive material 18 is shown in FIG. 4A. The conductivematerial 18 is preferably etched in a plasma, whose kind depends uponthe kind of the conductive material used. In the case of tungsten or ofan alloy of tungsten or of an alloy rich in tungsten, for example, aplasma of sulphur hexafluoride (SF₆) can be used. Since it is attemptedto expose entirely the surface of the separation layer 13 so that onlyelements 18a of the local conductive material within the contactopenings 16c are maintained, it is necessary to slightly prolong theattack in order to take into account possible tolerances of thethickness of the conductive material 18 or slight variations in theoperating conditions. In accordance with the kind of the separationlayer 13 and its more or less high resistance to this attack, the levelof the element 18a of conductive material can therefore be situatedslightly below the level of the separation layer 13 at the end of theetching step. The termination of the attack is defined either by anaccurate control of the etching time or by an arbitrary detection meansespecially utilizing the variation in intensity of a specific jetemitted by the plasma.

The following operation consisting in eliminating the separation layer13 is carried out selectively with respect to the insulating layer 12.Consequently, the surface of the insulating layer 12 thus exposed isdevoid of any irregularity independently of the possible roughness ofthe surface of the separation layer 13. Advantageously, the material ofthe separation layer 13 will be chosen so that it can be eliminatedselectively with respect to the conductive material 18. Thus, as shownin FIG. 5A, the upper level 20 of the element 18a of conductive materialmay be made higher, if desired, with respect to the level 21 of thesurface of the insulating layer 12. Such a shift is obtained bydifferences with respect to the thickness of the separation layer 13,which is eliminated.

According to a first practical embodiment of the invention, theseparation layer 13 is made of silicon nitride (Si₃ N₄). At the end ofthe attack of the layer of conductive material 18, the separation layer13 does not exhibit an etching barrier, but is on the contrary attackedat a rate which is of the same order as the rate of attack of theconductive material 18 in the case in which this material is tungsten oran alloy rich in tungsten. A thickness of the separation layer 13 has tobe chosen which corresponds to the time for which the etching of thelayer of conductive material 18 is prolonged because a superficial partof the separation layer 13 will be etched during this prolongation ofthe etching treatment. By way of example, a thickness of 150 to 300 nmfor the separation layer 13 of Si₃ N₄ has proved to be suitable. Sincesilicon nitride can be etched selectively both with respect to silicaglass and with respect to tungsten while wet wet etching with hotphosphoric acid, such a method is used for removing the separation layer13 and producing in this manner the level difference indicated in FIG.5A between the top 20 of the element 18a conductive filling material ofthe contact opening and the level 21 of the insulating layer 12. Bycontrolling on the one hand the time of prolongation of the etchingtreatment of the layer of conductive material 18 and on the other handthe thickness of the separation layer 13, if can readily be ensured thatthe level 20 of the element 18a of conductive filling material is equalto the level of the surface 21 of the insulating layer 12 or that thislevel exceeds the latter level by a small given height. It has in factbeen found that in these conditions the most favourable results areobtained with respect to the contact resistance between the elements 18aof conductive filling material and the metallic interconnection layer 22subsequently deposited on the structure.

The metallic layer 22 is formed, for example, from aluminum or an alloyof aluminum-silicon in a thickness of approximately 1 μm, in which layerthe configuration of interconnections is formed by the conventionalphotomasking and etching methods.

According to another embodiment of the invention, the material used forforming the separation layer 13 is of such a kind that the conductivematerial 18 can be etched selectively with respect to this separationlayer 13. In this case, use is preferably made of aluminum or an alloyof aluminum or of cobalt. At the stage of the method consisting inremoving by etching the major part of the conductive material 18, theseparation layer 13 then constitutes a blocking layer at the end of thisetching step. When the attack of the conductive material 18 isprolonged, the separation layer 13 is not attacked. Therefore, thethickness chosen for this layer can be slightly smaller than in thepreceeding embodiment. Thus, a thickness chosen to lie between 50 and150 nm is particularly suitable. When the separation layer 13 is formedfrom the aluminum or from an alloy of aluminum, it can be removedselectively with respect to the insulating layer 12 and with respect tothe conductive filling material 18 by means of dry etching in a plasmarich in chlorine ions. It can also be realized by wet etching in amixture of phosphoric acid, acetic acid and nitric acid frequently usedby those skilled in the art.

When the separation layer 13 is made of cobalt, in order to eliminateselectively this layer, a wet etching treatment is carried out.

According to a particularly advantageous embodiment of the invention,the selectively of etching the separation layer 13 with respect to theinsulating layer 12 is utilized during the step of etching the contactopenings 16c. In fact, etching conditions can be chosen in theinsulating layer 12 which are such that the separation layer 13 is notattacked and thus serves as an additional mask for etching the contactopenings 16c. When the separation layer 13 is made of aluminum or ofcobalt and, after having formed an opening 16b in this layer while usinga suitable etching step in the presence of the photoresist mask 15 (seeFIG. 2), the openings 16c are etched through the insulating layer 12. Asindicated above, this etching step is preferably effected by reactiveion etching, the ions employed being rich in cloride, for which etchingstep the separation layer 13 with its opening 16b constitutes a maskhaving a higher resistance to this etching step than the photoresistmask 15 itself. The latter may be removed before the contact openingsare etched, but it may also be maintained. By using a separation layer13 as an additional mask for etching the contact openings 16c into theinsulating layer 12, the step of etching narrow and deep contactopenings like the formation of practically vertical walls of openings isfacilitated.

FIG. 3B shows a variation of the embodiment of the method according tothe invention. In this variation, the contact openings 16b, 16c arefilled with a conductive material by first depositing a thin layer of atitanium-tungsten alloy as an adhesion and covering layer on the wholesurface, inclusive of the inner surface of the contact opening 16c, andthen by carrying out the deposition of a thick layer 28 of tungsten,which ensures that the contact openings are effectively filled. Asindicated above, the tungsten layer 28 is formed by low pressurechemical vapour deposition (LPCVD), while the layer of titanium-tungstenalloy 27 is deposited by cathode sputtering in a thickness of the orderof 100 nm.

FIG. 4B corresponds to a subsequent stage of the method equivalent tothat of FIG. 4A, in which the reference numerals corresponding to thesimilar elements are identical.

The major part of the conductive filling material 28 and the adhesionlayer 27 have been removed from the surface of the separation layer 13and only a part 27A and 28A located in the contact openings 16c isformed of these materials. The adhesion layer 27 of titanium-tungstenalloy is etched in the same conditions as the pure tungsten, one etchingstep following the other during the same plasma etchind operation. Theseparation layer 13 therefore plays the same part as that describedabove with reference to FIG. 4A. FIG. 5B illustrates the subsequentstage of the method, which corresponds to that of FIG. 5A, in which theseparation layer 13 has been removed, while a metallic interconnectionlayer 22 is disposed on the structure and is in contact with the localelement of conductive material 28a, 27a. As described above, the removalof the separation layer 13 allows again the level 20 of the element 28aof the conductive filling material to be raised with respect to thelevel 21 of the insulating layer 12. In the variation described withreference to FIGS. 3B and 5B, the separation layer 13 can be formed fromone of the materials already mentioned above, i.e. silicon nitride,aluminum, an alloy of aluminm or of cobalt.

The method according to the invention is not limited to the manufactureof a structure of interconnections on an integrated circuit comprising aMOS transistor; it has more generally for its object to form contacts onany kind of semiconductor device when contact zones and contact openingsof very small dimensions should be used.

We claim:
 1. A method of manufacturing a configuration ofinterconnections on a semiconductor device, said method comprising:(a)forming an insulating layer on a substrate, provided with saidsemiconductor device, (b) providing said insulating layer with aseparation layer capable of being etched selectively as to form openingscorresponding to contact openings to be etched into said insulatinglayer, (c) selectively etching said openings corresponding to saidcontact openings into said separation layer, (d) etching said narrowcontact openings into said insulating layer, (e) depositing at least onelayer of conductive material, having an overall thickness which issufficient to fill the volume of said contact openings, on saidseparation layer, (f) selectively removing, by etching the major part ofsaid conductive material and said underlying separation layer to therebyexpose the surface of said insulating layer while maintaining saidconductive layer and said operation layer in said contact openings, (g)selectively removing said separation layer from said contact openings,and (h) depositing a metal interconnection layer on said insulatinglayer and etching said metallic interconnection layer into a desiredconfiguration.
 2. A method as claimed in claim 1, characterized in thatthe separation layer is chosen so that it can be eliminated selectivelywith respect to the conductive filling material.
 3. A method as claimedin claim 2, characterized in that the insulating layer is formed from asilica glass, the conductive filling material is formed from tungsten oran alloy rich in tungsten and the separation layer is formed fromsilicon nitride.
 4. A method as claimed in claim 2, wherein theseparation layer is chosen of the materials such that the conductivefilling material can be etched selectively with respect to theseparation layer and the separation layer is employed as anetch-stopping layer in the step in the method wherein the major part ofthe conductive material is etched to expose the surface of theinsulating layer while the conductive material is maintained in thecontact openings.
 5. The method of claim 4, wherein during the step ofetching the contact openings the separation layer is etched selectivelyand is used as an additional mask for etching said insulating layer. 6.A method as claimed in claim 5, characterized in that, the conductivefilling material is formed from tungsten or an alloy rich in tungstenand the separation layer is formed from one of the metals: aluminumalloy or cobalt.